The present invention relates to a method for operating an SCR catalytic converter provided for the after treatment of exhaust gases of an internal combustion engine. The SCR catalytic converter is operated with a reagent which is introduced upstream of the SCR catalytic converter or a precursor of the reagent, wherein the actual NOx concentration downstream of the SCR catalytic converter is monitored on the basis of the signal of a sensor, which is arranged downstream of the SCR catalytic converter and which is sensitive at least with respect to NOx, in comparison with a calculated NOx concentration on the basis of modeled values.
The invention also relates to a computer program and a computer program product which are provided for carrying out the method.
Methods and devices are known for operating an internal combustion engine in particular in motor vehicles, in the exhaust-gas region of which there is arranged an SCR (selective catalytic reduction) catalytic converter by means of which the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine are reduced in the presence of a reagent to form nitrogen. The content of nitrogen oxides in the exhaust gas can be considerably reduced in this way. It is possible for ammonia (NH3) or formic acid, for example, to be provided as reagent for the reaction process. The reagent or a precursor is admixed to the exhaust gas upstream of the catalytic converter, by virtue of for example NH3, either NH3 directly or reagents which split to form NH3, preferably a urea-water solution, being used as reagent. The urea-water solution is injected into the exhaust tract upstream of the SCR catalytic converter by means of a dosing device. From said solution there is formed NH3 which acts as a reagent or as a NOx reducing agent. The demand for reagent is dependent on the untreated NOx emissions % NOx−Roh of the engine and thus in particular on the present rotational speed and the torque of the internal combustion engine. The dosing is therefore carried out preferably as a function of characteristic operating variables of the internal combustion engine and as a function of characteristic exhaust-gas variables.
The dosing of the reagent must be carefully defined. In the case of excessively low dosing, the nitrogen oxides can no longer be completely reduced in the SCR catalytic converter and are released as emissions. In the case of excessively high dosing, so-called reagent slippage (NH3 slippage) may occur, which firstly leads to an unpleasant smell as a result of the NH3 that is released, and secondly results in an unnecessarily high consumption of reagent.
An SCR catalytic converter has storage characteristics for the reagent. The stored NH3 amount is also referred to as the reagent fill level of the SCR catalytic converter. Here, the reagent fill level denotes the degree of loading of the SCR catalytic converter with stored reagent. The reagent storage behavior is dependent on the respective operating temperature of the SCR catalytic converter. The lower the temperature, the greater the storage capability.
The efficiency of an SCR catalytic converter is dependent both on the temperature and also on the reagent fill level. With an increasing reagent fill level, the efficiency increases.
If it is sought to attain the highest possible NOx conversion rates, it is necessary to operate the SCR catalytic converter with a high reagent fill level. If the maximum reagent fill level is reached, reagent slippage may briefly occur in the event of load steps or under transient conditions of the internal combustion engine, even in the case of a very precisely configured dose amount. This may be the case even when reagent is no longer being dosed. In general, this is tolerable in order to achieve a high NOx conversion target.
The calculation of the required amount of reagent is subject to a multiplicity of errors and deviations. For example, the calculation of the dose amount is influenced by the untreated NOx emissions % NOx−Roh, the efficiency of the SCR catalytic converter and inaccuracies of the dosing system itself. An incorrectly calculated reagent fill level in the SCR catalytic converter may therefore arise even in the case of a new, carefully applied system. In general, an adaptation of the reagent fill level is necessary. For this purpose, use is normally made of a NOx sensor which is arranged downstream of the SCR catalytic converter and which detects the amount of nitrogen oxide downstream of the SCR catalytic converter. Owing to the measurement principle in the case of conventional NOx sensors, said sensors generally exhibit cross-sensitivity with respect to the reagent NH3. This is referred to as a reagent cross-sensitive, for example an NH3 cross-sensitive, NOx sensor. A conventionally used NOx sensor thus measures a summed signal of NOx and NH3.
The SCR catalytic converter model used for an adaptation dynamically calculates the NOx emissions downstream of the SCR catalytic converter. Deviations from the measured NOx sensor values may have various causes: aside from the model inaccuracy (up to ±50 ppm) as a first cause, the causes of the deviation may be an underestimation of the reagent fill level in the SCR catalytic converter and thus a release of NH3 (reagent slippage), or an overestimation of the reagent fill level and thus a conversion deficit and a release of NOx (NOx slippage).
To optimize the dosing of the reagent, the laid-open specification DE 10 2004 031 624 A1 proposes a method for operating an SCR catalytic converter used for the purification of the exhaust gas of an internal combustion engine control or regulation of the reagent fill level in the SCR catalytic converter to a predefined setpoint value. The theoretical reagent fill level in the SCR catalytic converter is determined on the basis of a catalytic converter model (SCR model). By means of the targeted specification of the setpoint value, it is intended to ensure that, in particular in transient states of the internal combustion engine, firstly there is an adequate amount of reagent available for the catalytic reduction of nitrogen oxides, and secondly reagent slippage is avoided. Said method is suitable for compensating drifts or slow changes in the behavior of the SCR catalytic converter. For a compensation of very large and spontaneous changes, for example in the event of tank replenishment with an incorrect reagent (for example water), such methods lead to only unsatisfactory results, because the regulation times are very long and last for several hours, for example.
The laid-open specification DE 10 2010 002 620 A1 describes the setting of a long-term adaptation factor. The long-term adaptation factor has a direct influence on the pilot-control amount of the reagent. The dosing strategy thus adapts to the respective system and to longer-term environmental influences and can thus reduce the number of adaptation interventions necessary in the case of systematic errors. The above-mentioned problem cannot be solved through the use of long-term adaptation factors either, because here, too, the adaptation to large and spontaneous changes is very inert.
It is, by contrast, the object of the invention to be able to react very quickly to dose amount deviations which lead to inadequate exhaust-gas after treatment. Here, it is sought in particular to provide an immediate measure for being able to optimally operate the exhaust-gas after treatment system.